1. Field of the Invention
The present invention relates to an optical scanner the principal use of which is in recording or forming an electrostatic latent image on the surface of a photoreceptor as a medium to be scanned in image forming apparatus typified by laser printers and copiers.
2. Description of the Related Art
Scanning lens systems in optical scanners extensively use plastic lenses to achieve cost reduction. Since plastics have a negative temperature coefficient of refractive index, a shift in the position of image formation occurs if the temperature of the environment changes and this is problematic if the scanning lens systems are to be used in optical scanners that require high precision. With a view to solving this problem, various structural designs have so far been proposed for such high-precision optical scanners.
For example, Unexamined Published Japanese Patent Application (kokai) No. 273463/1993 proposes that a single aspherical glass lens be used as a collimator lens. All that is done to correct the shift in the position of image formation by the scanning lens system due to temperature changes is by changing the back focus of the glass collimator lens as a result of index changes and by effecting changes in the distance between a semiconductor laser and the collimator lens as a result of thermal expansion of their holder. However, the correction that can be accomplished is not sufficient to justify application to high-precision optical scanners.
Unexamined Published Japanese Patent Application (kokai) No. 341215/1993 proposes the use of a single plastic cylindrical lens having negative power in an sub-scanning direction which is perpendicular to the principal scanning plane. The working theory of this proposal is cancelling the change in the negative power of the cylindrical lens in the sub-scanning direction by the change in the positive power of the scanning lens system in the sub-scanning direction, so it is not effective for correcting the shift that occurred in the position of image formation in the main scanning direction due to the change in the positive power of the scanning lens system.
According to Unexamined Published Japanese Patent Application (kokai) No. 341216/1993, a plastic lens of rotation symmetry having negative power is provided in the collimator lens and an anamorphic plastic lens having positive power in the main scanning direction is provided in the cylindrical lens. The working theory of this proposal is cancelling the change in the negative power of the plastic lens of rotation symmetry in the collimator lens system by the change in the positive power of the plastic lens in the cylindrical lens, so it is not effective for correcting the shift that occurred in the position of image formation in the main scanning direction due to the change in the positive power of the scanning lens system. As a further problem, incorporating two additional plastic lenses having power increases the chance of misalignment occurring during lens assembly or due to changes in the temperature of the environment; this precision problem makes the proposed structure undesirable from the viewpoint of application to optical scanners of high precision.
According to Unexamined Published Japanese Patent Application (kokai) No. 341217/1993, both a semiconductor laser and a collimator lens are mounted in a plastic barrel, which deforms as a result of changes in the temperature of the environment to effect changes in distance. However, there is no assurance that the temperature of the barrel changes uniformly in every part of it; in addition, the barrel is more complex in shape than spheres and flat plates and prone to misalignment, so the precision of the proposed structure so is not high enough to justify its application to optical scanners of high precision.
An object, therefore, of the invention is to provide a highly precise and low-cost optical scanner that uses plastic lenses in a scanning lens system and which effectively suppresses the shifts in the position of image formation that occur as a result of changes in the temperature of the environment, thereby making it viable with increased density of dots on recording media.
This object of the invention can be attained by an optical scanner comprising a coupling lens system for collimating rays of light from a light source, a light deflector for deflecting said rays of light for scan and a scanning lens system for focusing the deflected rays of light to form an image on a medium being scanned, said coupling lens system including a first optical element as at least one optical element other than the optical element located the farthest away from said light source, said first optical element having substantially no power and being made of a material having a negative temperature coefficient of refractive index, said scanning lens system including at least one second optical element made of a material having positive power and a negative temperature coefficient of refractive index.